Disturbed daily rhythms have been implicated in a variety of brain disorders. These rhythms are regulated by at least two intertwined pathways, one is the circadian clock and a second is the masking pathway that mediates activity in response to light. This proposal addresses these pathways using the fruit fly, Drosophila melanogaster. The genetic basis of circadian and masking function appears conserved, suggesting that findings in the fly will be widely applicable. A critical role for an enigmatic and highly conserved ion channel narrow abdomen (na) has been identified in both circadian output and masking regulation. These observations underlie the overall goal of this proposal, which is to provide an integrated view of na function in both masking and circadian rhythms at the behavioral, neuronal, and molecular levels. The specific aims of this proposal are: 1. To map the neural substrates of na function. NA may function in circadian pacemaker neurons to regulate masking behavior. Tissue-specific rescue and assessments of NA distribution will be performed to address NA function in circadian and photoreceptor neurons. The regulation of NA by the clock and/or light will also be examined. 2. To define the electrophysiological phenotype of na mutant neurons. A method has been developed to examine the electrical properties of Drosophila pacemaker neurons. Characterization of ionic currents in these key neurons, the effects of na on these currents, as well as the electrophysiological properties of the elusive NA channel itself will be examined, including regulation by the circadian clock and/or light. 3. To assess the consequences of altered NA channel properties. NA is a unique and conserved member of the voltage-gated cation channel family. NA reveals many signatures of ion channels including highly conserved pore selectivity and voltage sensor sequences. The function of NA channels with altered pore or voltage sensor sequences will be tested by behavioral rescue. In total, these studies will define the electrophysiological function of NA within distinct anatomic pathways relevant to circadian and masking regulation. Given the conservation of clocks and NA, this proposal should illuminate human daily rhythms and their contribution to disease.